This invention relates to a fluorescent display device, and more particularly to a fluorescent display device wherein a face plate is formed thereon with a pseudo half mirror.
Now, a conventional fluorescent display device including a half mirror will be described with reference to FIGS. 4(a) and 4(b). The conventional fluorescent display device includes an anode-side substrate 11 made of glass. The anode-side substrate 11 is formed thereon with anode electrodes 12 each having a phosphor deposited thereon. Also, the conventional fluorescent display device includes grids 13 arranged above the anode electrodes 12 in a manner to be spaced from each other at predetermined intervals and from the anode-side substrates 11 at predetermined intervals. Further, the fluorescent display device includes filamentary cathodes 14 arranged above the grids 13 in a manner to be spaced from each other at predetermined intervals and from the grids and therefore the anode-side substrate 11 at predetermined intervals. Reference numeral 21 designates a face plate made of glass. The face plate 21 is formed on a rear surface thereof with a chromium film 22. The fluorescent display device also includes side plates 31 and 32 made of glass. The anode-side substrate 11, face plate 21 and side plates 31 and 32 thus arranged are sealedly joined to each other by means of sealing glass materials 33 and 34, to thereby constitute a vacuum envelope.
The chromium film 22 acts as a half mirror, which functions to improve contrast of display as in a neutral density filter. Also, the half mirror exhibits both an electrostatic shielding function and an electron diffusion function. Such functions are disclosed in Japanese Utility Model Application Laid-Open Publication No. 108646/1980.
The chromium film 22 shown in FIGS. 4(a) and 4(b) is constructed so as to permit light transmittance thereof to be varied depending on a thickness thereof. For example, the thickness of 100 Å permits the chromium film 22 to exhibit light transmittance of about 10% and that of 190 Å permits it to exhibit transmittance of 0%. Thus, the thickness of 190 Å keeps the chromium film 22 from transmitting light therethrough. Therefore, formation of the chromium film 22 into a half mirror requires to reduce a thickness of the chromium film 22 to a level of 190 Å or less. Light transmittance of the chromium film 22, as described above, is varied depending on a thickness thereof. Thus, substitution of the chromium film 22 for the neutral density filter requires to form the chromium film 22 into a thickness which permits it to exhibit light transmittance at a predetermined level. However, light transmittance of the chromium film 22 is substantially varied depending on a slight variation in thickness thereof, thus, the thickness must be accurately controlled at a level as small as Å. This causes formation of the chromium film 22 to be highly troublesome. Also, manufacturing of the fluorescent display device requires a calcination step of heating it to a temperature as high as 400° C. or more. However, this causes oxidation of the chromium film 22 during the calcination, to thereby reduce light transmittance thereof.
Thus, the prior art not only renders control of the film thickness and formation of the film troublesome or hard, but causes a reduction in light transmittance of the film due to oxidation thereof during the calcination, resulting in manufacturing of the chromium film which has desired light transmittance being hard.
The chromium film 22 fails to exhibit conductivity when a thickness thereof is reduced to a level of 40 Å or less. When the chromium film 22 has a thickness which permits it to function as a half mirror, it is increased in resistance, so that it fails to satisfactorily exhibit an electrostatic shielding function and an electron diffusion function. Also, the chromium film 22 is formed into a very small thickness. This, when a contact lead is pressedly contacted with the chromium film 22, causes a portion of the chromium film 22 contacted with the lead to be damaged, so that electrical connection therebetween may be often deteriorated. Chromium for the chromium film 22 is harmful, resulting in handling of chromium and a treatment thereof during manufacturing of a fluorescent display device being troublesome, leading to a failure to facilitate manufacturing of the fluorescent display device and an increase in manufacturing cost thereof. Further, chromium is expensive to a degree sufficient to significantly increase a manufacturing cost of the device. Also, disposal of the fluorescent display device after a life thereof ends is in danger of causing environmental pollution by chromium.
In the conventional fluorescent display device, the chromium film 22, as shown in FIG. 4(b), is formed on only a portion of the rear surface of the face plate 21 arranged in the vacuum envelope, to thereby be kept from being formed on an area between the sealing glass material 33 and the face plate 21. Thus, when the face plate 21 is viewed from an outside of the fluorescent display device, an adhesion area formed by the sealing glass material 33 is observed as an architrave-like frame around a display section of the fluorescent display device, so that the display section is observed in a size smaller than an actual size thereof. Also, the chromium film 22 is caused to exhibit a blue to green color due to a high temperature at which it is exposed during calcination of the fluorescent display device. Such discoloration is disadvantageously amplified when an equipment such as an acoustic equipment, an image display equipment or the like on which the fluorescent display device is mounted has metallic finish or exhibits a metallic feeling, leading to a deterioration in a harmony of colors or design between the equipment and the fluorescent display device.